With only 3 non-overlapping channels, and often wifi access points choosing their own overlapping channel (like 3 or 8), your parent's wireless is likely interfering with a neighbor's wireless. This is much more likely in an apartment complex.
If someone is running 802.11g (or, 802.11b because they only have 6mbps DSL and 11mbps 802.11b is more than enough for their DSL), they are occupying the wireless channel for an extended amount of time.
Even a group of grandmas in an apartment complex running 802.11b only to access their 6mb DSL connection would quickly see their speeds plummet because of CMSA/CA causing a cascade failure of the wireless signal.
Going with the current wireless standard (802.11n in both 2.4 and 5ghz) is the right answer. 802.11ac is very new, so I would agree for now that the additional cost isn't worth it. At the very least, 802.11n 2.4ghz should be default.